This invention relates to a positioning device for supporting an object by magnetic forces without contacting the object, and positioning the object by controlling the magnetic forces.
A mechanical positioning device is known which can accurately position an object, using a ball screw. The ball screw is rotated, thus moving the base on which the object is mounted. The object is therefore positioned in accordance with the rotational angle of the ball screw. To accurately position the object the ball screw must be machined with high precision. However, since is a limitation to the precision of the screw, it is difficult to obtain highly accurate positioning of the object. Maintenance and inspection must be periodrically made on the mechanical positioning device to maintain the positioning accuracy for years. It is improper to incorporate such a mechanical positioning device into an apparatus which cannot be inspected for a long time.
A positioning device utilizing an electromagnet has been proposed to replace the mechanical positioning device. This new device has a base on which an object is mounted. The base is supported and moved in non-contact state by magnetic force generated by the electromagnet thus positioning the object. The magnetic force is controlled to position the object. The magnetic force necessary for supporting the base at a predetermined position is inversely proportional to the square of the distance between the electromagnet and base. Therefore, in order to move the base, and, thus, the object, to a predetermined position, the non-linear magnetic forces generated by the electromagnet must be linearized. In other words, it is necessary to convert a current supplied to the electromagnet into a non-linear current. Therefore, an electric circuit including not only the electromagnet, thus but also a device for such current conversion must be utilized. The use of such a complicated circuit inevitably reduces the reliability of the positioning device.